take/authorize: kill-switch + validate fund-claim amounts

The /take fund-claim path could pay out unbacked balance: the only ceiling on a
take is sats(fiat / rates[currency]) from the authorization, and a
crypto-denominated authorization (rate ~1) or a non-finite/zero rate turns that
ceiling astronomically large, so a take is no longer bounded by the authorized
fiat value. Combined with the fund debit/credit running per-call, an authorizer
account could be drained far beyond intent.

- Add a `fund:disabled` redis kill-switch to both /authorize and /take that
  fails the fund/authorize/take mechanism closed while it is audited.
- Reject non-finite/zero rates, non-fiat (crypto) authorization currencies, and
  non-positive/NaN fiat and amounts, so the fiat cap can no longer be defeated.
- Require the authorizer account to exist before debiting it.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01XiwgmQKwKnRxUmVDd4xSf7

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